The invention relates to analog buffers, and more particularly, to unit-gain analog buffers with a precise gate to source voltage (Vgs) compensation and a small DC offset.
In conventional display systems, digital-to-analog conversion (DAC) is the most important part of a driving circuit. Typically, a digital-to-analog converter requires a unit-gain analog buffer to improve the driving performance thereof. In normal integrated circuits (ICs) designs, analog buffer typically comprises operational amplifiers, as shown in FIG. 1. However, it requires a large chip area.
To reduce total module costs, the “system-on-glass” technology by low temperature poly-Si (LTPS) TFTs offer a promising solution because the LTPS allows integration of driving circuit on glass. However, operational amplifiers composed of TFTs have poor performance, such as small gain, large DC offset, high power consumption, slow slew rate, and large area.
FIGS. 2A and 2B show conventional analog buffers comprise N-type source follower or P-type source follower. The output voltage thereof, however, has a DC offset of a Vgs. FIGS. 3A and 3B show conventional analog buffers comprise a source follower with threshold voltage compensation and Vgs compensation. FIG. 3C is a timing chart of the conventional analog buffers shown in FIGS. 3A and 3B. It, however, also has a large DC offset at the output voltage and/or poor driving performance.